1. Field of the Invention
This invention generally relates to electrochemical devices such as fuel cells, and relates more particularly to improvements in solid oxide fuel cells (SOFCs) of the flat plate design, as well as methods for manufacturing the cells.
2. Background of the Invention
Fuel cells are electrochemical energy conversion devices that generate electricity and heat by converting the chemical energy of fuels. Solid-oxide fuel cells (SOFCs) are made from solid-state materials, such as ceramic oxide. SOFCs consist of three components: a cathode, anode, and an electrolyte sandwiched between the cathode and anode. Oxygen from the air is reduced at the cathode and is converted into negatively charged oxygen ions. These ions travel through the electrolyte to the anode, where they react with fuel, such as hydrogen. The fuel is oxidized by the oxygen ions and releases electrons to an external circuit, thereby producing electricity. The electrons then travel to the cathode, where they release oxygen from air, thus continuing the electricity-generating cycle. Individual cells can be stacked together in series to generate larger quantities of electricity.
Generally, SOFCs that employ planar cell units present design challenges because of a need to separate the air from the fuel by a seal substantially around the entire edge of the ceramic fuel cell plate. Also, the interconnection of stacks in a fuel cell assembly is made difficult because of the relatively high operating temperatures of SOFC which have ceramic, rather than metallic interconnects. Metallic interconnections are subject to oxidation, which leads to a loss of conductivity. Finally, the high operating temperature of SOFCs presents a further design challenge from the standpoint of the mechanical integrity of the fuel cell stack. When brought up to operating temperature and then back to room temperature, the fuel cell stack experiences dramatic thermal and mechanical stresses, which can lead to mechanical fatigue and failure, particularly where the fuel cells must be thermally cycle many times. The design problems discussed above are exacerbated when SOFCs are used in automotive applications such as auxiliary power units (APU) for vehicles. The automotive environment is particularly challenging and demanding, compared to the use of SOFCs in stationary applications, because of the need for higher power densities dictated size constraints, impact on fuel economy and emissions, crash worthiness, and deep thermal cycling over many cycles of use.
Several configurations for SOFCs have been developed, including monolithic, planar and tubular. The monolithic SOFC design is characterized by a honeycomb construction that is fused together into a continuous structure. Planar stacks, which have good energy densities, suffer from the fact that they require large perimeter seals around the entire edges of the ceramic fuel cell plates. Neither of these design elements lend themselves to rapid or uneven thermal cycling. Planar stacks need long, slow heat up cycles which is inconsistent with automotive applications where SOFCs are called upon to operate “on demand”. Tubular SOFCs require sealing only at the ends of the tubes over a relatively small area. The tubular seals are therefore distant from the hottest area of the fuel cell stack, consequently tubular SOFCs can be thermally cycled faster, and for more cycles. Unfortunately, tubular SOFCs exhibit markedly lower power per unit volume, compared to flat plate SOFCs, because their physical geometries do not allow high density, close stacking of the individual tubular units.
Accordingly, there is a clear need in the art for improved SOFCs that exhibit exceptionally high power per unit volume, which can not only withstand stresses stemming from deep thermal cycling over many cycles of use.